1. Technical Field
The present disclosure relates to a method for realizing a thin film organic electronic device integrated on a substrate comprising an organic material layer and at least an organic thin film transistor or OTFT transistor. The disclosure also relates to a thin film organic electronic device.
2. Description of the Related Art
As it is well known, in the last decade great interest has been shown in the field of the microelectronics, for organic electronic devices which use, alternatively with respect to some frequently used materials, organic material layers and in particular organic polymers.
Some examples of known devices are organic LEDs or OLEDs and OTFT transistors (Organic Thin Film Transistors).
The organic polymers allow to combine the high conductivity, normally property of the metals, with the mechanical properties of the organic materials, such as the flexibility and the possibility of being manufactured as thin films.
Moreover, organic electronic devices are, in general, cheaper than the traditional ones and show a high reliability and efficiency.
A particular interest has been also turned to the so called conjugate organic polymers which, from the electronic viewpoint, behave as insulators or semiconductors but which advantageously change the conductivity degree drastically according to the oxidation status.
For example, a conjugate organic polymer is subjected to a “doping” process step for increasing the conductivity by several orders of magnitude. The “doping” step or protonation step generally consists in a reaction of the conjugate polymer with a doping agent, oxidizing or reducing or a protonic acid, whereby a high polycationic or polyanionic delocalization is obtained.
A conjugate organic polymer, insulating or semiconductive, with a conductivity typically from 10−10 to 10−5 S*cm−1 subjected to the protonation step can increase the conductivity up to 1*104 S*cm−1. The polymer conductivity can be controlled by the nature of the doping agent, by the doping level and by the blending with other conventional polymers such as for example the insulators which allow to optimize the properties of the polymer obtained.
However, the main part of the conjugate organic polymers have a high ionization potential, about 5 eV, and thus use contact terminals with low ohmic contact, for example of the metallic type, to reach high performances of the organic devices obtained.
Metals suitable for this function are the noble metals, such as gold or platinum, which are however excessively expensive.
Alternatively, the prior art proposes the use of an interposed material which acts as interface between the metallic layers of a device, for example in the case of an OLED i.e., a LED of the organic type, between the anode and the cathode.
Efficient improvements have been obtained by using the polyaniline as interposed material, as reported in the articles by P. Vacca, M. G. Maglione, C. Minarini, G. Salzillo, E. Amendola, D. Della Sala, A. Rubino, entitled: “PANI-CSA: An Easy Method to Avoid ITO Photolithography in PLED Manufacturing”, Macromolecular Symposia; and by Y. Cao, G. M. Treacy, P. Smith, A. J. Heeger, entitled: “Solution-Cast Films of Polyaniline: Optical-Quality Transparent Electrodes”, Appl. Phys. Lett. (1992), Vol. 60, Pg. 2711; as well as in the document App. Phys. Lett. Vol. 73, pages 108-110; and in the U.S. Pat. No. 5,620,800 to Philips Corporation.
Particular interest has been turned also to the polyaniline doped and complexed with the camphorsulphonic acid, also known with the acronym PANI-CSA. In particular, it is known that the polyaniline PANI/CSA has a conductivity that can be modified by means of photolithographic techniques. According to a known process, described in Progr. Polym. Sci, Vol. 23, pages 993-1018 and schematically indicated in FIG. 1, by starting from a base of emeraldine, as semiconductor material, and by adding camphorsulphonic acid, as primary dopant (protonation), in an m-cresol solvent, as secondary dopant, and by treating the mixture obtained in an ultrasonic bath for forty-eight hours at the temperature of 50° C., an emeraldine salt is obtained, i.e., a metal.
The conductive mixture obtained can be deposited as layer on a glass or plastic substrate, as shown in FIGS. 2 and 3, by means of an etching with ultraviolet radiations, for example with a wavelength equal to λ=250 nm, and by using a suitable mask, an alternation can be obtained of conductive areas, i.e., non-irradiated portions covered by the mask, with insulating areas, i.e., irradiated portions which, from emeraldine salt become leucomeraldine. This process is also described in App. Phys. Lett. Vol. 73, pages 108-110 ed.
Even more in particular, the irradiated portions that have become insulating undergo an expansion which substantially results in an increase of thickness, as shown in FIG. 3.
For an emeraldine salt layer with a thickness for example equal to 0.3 μm the resistivity of the conductive portions is equal to 104 Ω/m2 while the resistivity of the non-conductive portions is equal to 1010 Ω/m2.
It is known that suitable polyaniline layers deposited on glass or plastic substrates behave as an anode for organic LEDs or OLEDs, as described in the article by G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, A. J. Heeger, entitled: “Flexible Light-Emitting Diodes Made From Soluble Conducting Polymer”, Nature (1992), Vol. 357, Pg. 477, and also in the articles by Y. Yang, E. Westerweele, C. Zhang, P. Smith and A. J. Heeger, entitled: “Enhanced Performance of Polymer Light-Emitting Diodes Using High-Surface Area Polyaniline Network Electrodes”, J. Appl. Phys. (1995), Vol. 77, Pg. 694; and by Y. Yang, and A. J. Heeger, entitled: “Polyaniline as a Transparent Electrode for Polymer Light-Emitting Diodes: Lower Operating Voltage and Higher Efficiency”, Appl. Phys. Lett. (1994), Vol. 64, Pg. 1245.
Some schematic examples of organic LEDs or OLEDs, realized by the Applicant, are schematically shown in FIGS. 5, 6 and 7.
In particular, in the example of FIG. 5, the LED shows a multilayer structure which comprises a thin oxide layer of indium, also known with the acronym of ITO (Indium Tin Oxide), realized on a substrate, in particular of glass; on the ITO layer a thin film of poly(N-vinylcarbazole) or PVK, a layer of Alq3 (Tris (8-hydroxy) quinoline aluminum)—serving as electrons carrier layer and also indicated as ETL (acronym from the English: “Electron Transport Layer”)—and an upper aluminium layer are realized in sequence. The substrate realizes the anode of the LED thus obtained, while the aluminium layer realizes its cathode.
In the example of FIG. 6, the multilayer structure suitable for realizing the organic LED differs from the example of FIG. 5 due to the presence of a polyaniline layer PANI/CSA interposed between the layer ITO and the layer PVK.
Further, in the example of FIG. 7, the multilayer structure is substantially similar to the structure of FIG. 6 where the layer of PVK is replaced by a layer of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine or TPD.
The interposition of the polyaniline layer PANI/CSA allows, advantageously, to lower the voltage barrier between the anode and the cathode of the OLED with an overall improvement of the technical features as shown in the diagrams of FIGS. 8 and 9, referred to the OLED realized according to FIG. 6, which show a substantial improvement in the charge transport which allows to reduce the turn-on or barrier voltage, as further indicated in the comparative table of FIG. 10 with reference to the example of FIGS. 5-7.
This table shows, in particular, that from a barrier voltage of 0.9 eV of the OLED of FIG. 5 a voltage equal to 0.3 eV of the OLED of FIG. 7 is reached.
It is to be underlined that the combination between the high conductivity of the polyaniline with the low surface resistance of the layer of ITO is responsible for the effective improvement of an electrode realized with a double layer PANI/ITO, as shown in FIGS. 6 and 7 with respect to one realized with a single layer of ITO, as shown in FIG. 5 or similarly, if a single polyaniline layer were used.
Further, it is to be underlined that the polyaniline layer operates as a “buffer” layer between the ITO layer and the overhanging conjugate polymer, realized by the layer PVK or by the layer TPD, allowing, even more in particular, to control the release of oxygen from the ITO layer caused by the oxidation of the conjugate polymer.
In fact, it is known that the oxygen released from the ITO layer determines a degradation mechanism of the overhanging activated conjugate polymer layer, as also described in the article by S. Karg, J. C. Scott, J. R. Salem, M. Angelopoulos, entitled: “Increased Brightness and Lifetime of Polymer Light-Emitting Diodes With Polyaniline Anodes”, Synthetic Metals (1996), Vol. 80, Pg. 111.
An organic electronic device, realized by the Applicant according to the principles of the prior art, is schematically shown in FIG. 12, globally indicated with 1.
In particular, the device 1 is integrated on a substrate 10, of glass or of plastic material, and in the embodiment shown it comprises an organic thin film transistor OTFT (organic thin film transistor), indicated with 90, as well as a circuitry section 95, wherein at least one contact area PAD is realized for the external connection by means of contact electrodes with supply voltage references and/or data transfer circuits.
The OTFT transistor 90 is in a top-gate/bottom-contact arrangement and comprises source 16 and drain 17 contact areas.
The source 16 and drain 17 contact areas and the contact area PAD are suitably realized from a first conductive organic material layer 5 deposited on a substrate 10.
The device 1 also comprises a second filling dielectric layer 11 realized on the substrate 10.
Above the source 16 and drain 17 contact areas and in contact with them, the device 1 comprises a third semiconductor organic material layer 12 used to improve the electric qualities of the organic transistor OTFT 90.
A fourth dielectric material layer 13 completely covers the substrate 10 leaving the contact area PAD 95 exposed.
Above the source 16 and drain 17 contact areas a fifth conductive organic material layer 14 is further present suitable for realizing a gate contact area for the transistor OTFT 90.
A sixth encapsulation layer 15 completely covers the substrate 10 leaving only the contact are PAD exposed for the external connection with the device 1 obtained.
A known method for realizing the device 1 is hereafter described with reference to FIGS. 13 to 24. This method has also been shown during the conference “Printed Electronics Conference” in San Francisco on Nov. 13th-14th, 2007 (by Mr. L. Occhipinti).
The method comprises the steps of:                depositing on the substrate 10 of glass or of plastic material, with spin coating procedure the first conductive organic material layer 5;        patterning this first layer 5 by means of an etching with reactive ions or RIE (Reactive Ion Etching) to obtain a contact area PAD/Vias for external connections and source 16 and drain 17 contact areas for the transistor OTFT 90, as indicated in FIG. 13.        
The method then provides the steps of:                depositing on the substrate 10 a second filling dielectric layer 11. Advantageously, the second dielectric layer 11 is of the organic type with low dielectric DIE1 and covers the source 16 and drain 17 areas as indicated in FIG. 14;        patterning by soft-lithographic or imprinting procedure the second organic dielectric layer 11 by using a suitable mold Mold1, of the type shown in FIG. 4, used to transfer a predetermined pattern to said second organic dielectric layer 11.        
It is known that elastomeric molds thus made mainly have a flat structure with prearranged protuberances having a predefined geometric relation. In particular, L indicating the width of these protuberances and H their thickness, their relation is: 0.2<H/L<2
Even more in particular, the mold Mold1, in the present embodiment, shows protuberances above the contact area PAD, the source 16 and drain 17 areas, as shown in FIG. 15. The mold Mold1 will be explained further hereafter.
Subsequently, the method provides the steps of:                removing the mold Mold1, as shown in FIG. 16;        etching the second organic dielectric layer 11 to bring to surface the contact area PAD, the source 16 and drain 17 contact areas, as shown in FIG. 17 according to the patterning formed by the mold Mold1.        depositing above the second organic dielectric layer 11, in a selective and localized way, a third organic material layer 12.        
The method provides to deposit on the source 16 and drain 17 contact areas, as shown in FIG. 17, a semiconductor organic material 12a OSC1, and to deposit above the contact areas PAD, a conductive organic material 12b of the MTL2 type, as shown in FIG. 18.
Suitably, the deposition of the semiconductor organic material 12a OSC1 occurs by means of inkjet procedure or IJP (InkJet Printing) using an ink jet printer 50 by employing for example P3HT material (Poly (3-Hexylthiophene)) or other material, while the deposition of the conductive organic material MTL2 12b, occurs by means of IJP procedure by employing for example material of the Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT/PSS) type or metal that can be patterned.
The method provides then the steps of:                depositing with spin coating procedure a fourth layer 13 suitable for completely covering the substrate 10 as shown in FIG. 19; preferably the fourth layer 13 has high dielectric constant K;        patterning by means of soft-lithographic or imprinting procedure the fourth layer 13 by using a second mold Mold2 for carrying out suitable and prearranged reductions of thickness of the fourth layer 13 in correspondence with the contact areas PAD, as shown in FIG. 20.        
Subsequently, the method provides the step of:                removing the second mold Mold2;        etching the fourth layer 13 to bring to surface the contact area PAD, as shown in FIG. 21;        depositing with spin coating procedure a fifth layer 14 suitable for completely covering the substrate 10, as shown in FIG. 22; preferably the fifth layer 14 is a conductive organic material, for example PEDOT/PSS or a metal that can be patterned;        patterning by means of soft-lithographic or imprinting procedure the fifth layer 14 by using a third mold Mold3, as shown in FIG. 23 to define by means of etching suitable and prearranged gate contact areas for the transistors OTFT.        
Subsequently, the method provides the step of:                removing the third mold Mold3;        etching the fifth layer 14 to eliminate the residues of organic material;        depositing an encapsulating layer, not shown in these figures.        
Advantageous under several aspects, the organic devices thus obtained however show some drawbacks.
For connecting the organic polymers, contained in the OTFT devices, to supply sources and/or to circuits for the data transfer, contact terminals with low ohmic contact are used. As previously indicated, the ionization potential of the mainly used organic polymers is equal to about 5 eV.
A further drawback of the organic devices thus obtained is linked to the patterning step of the conductive organic layer, as described also in the article by Kymissis, Dimitrakopoulos, Purushothaman, entitled: “Patterning Pentacene Organic Thin Film Transistors”, J. Vac. Sci. Technol. B 20, pages 956-959 (2002).
A correct patterning step, in particular in the OTFT transistors obtained, considerably reduces the leakage current and thus increasing the ration ION/IOFF, as described in the article by Gelinck et al., entitled: “Flexible Active-Matrix Displays and Shift Registers Based on Solution-Processed Organic Transistors”, Nature Materials 3 pages 106-110 (2004).
Generally, in the process of realization of the organic devices the patterning step occurs by means of “wet” steps which however are not suitable for organic polymers with low molecular weight.
In fact, these organic polymers show a high intermolecular interaction but a weak interaction with the underlying substrate and the exposure of the organic polymer layer to liquid solvents induces a relaxation between the chemical bonds of the molecules which drastically reduces the conductive properties i.e., of charge transport. This occurs also if the organic polymer used is insoluble in the solvent used.
Several solutions have been proposed for the step of patterning the layers or thin films of organic polymers. None of them however is satisfactory for all the applications.
A solution provides to use a mask of the shadow mask type, other solutions proposed provide instead to physically remove the conductive organic polymer not necessary, for example by means of a lithographic step.
These solutions although satisfactory in a research lab show serious difficulties when used in a production in series and are also little suitable for producing the patterning for high areal densities.
Further, some of the known solutions require masks that must be cleaned any time they are used, while the masks with high resolution show the drawback of being very fragile.
To solve these drawbacks it is known to use so called printing methods which combine a deposition step and a patterning step in a single step.
A known solution is described in the article by Z. Bao, A. Dodabalapur, H. Katz, R. V. Raju, J. A. Rogers, entitled: “Organic Semiconductors for Plastic Electronics”, Bell Laboratories, Lucent Technologies.
However, the realization of OTFT transistors by means of the printing method strongly depends on the availability of starting materials of high quality and typically formulated according to a specific composition.
The realization by means of the printing method results thus rather complicated and expensive particularly not suitable for the realization of devices on a large scale.
An alternative solution provides the use of conventional photolithographic techniques.
The conventional photolithographic techniques however show some drawbacks, such as the example described in the article by Y. Xia and G. M. Whitesides, entitled: “Soft Lithography”, Angew. Chem. Int. Ed. 37, 550-575 (1998).
Moreover, the sizes generated by means of the photolithographic techniques are limited by the optical diffraction and to be easily applied, they require the presence of planar surfaces. In fact, their use on irregular surfaces is particularly complicated and expensive.
Further, the surfaces obtained by means of the patterning step are not chemically controlled, i.e., they could undergo modifications that cannot be foreseen to date.
Moreover, for generating small openings or “features” by means of photolithographic techniques radiations with high energy are required which however need particularly complex plants and technologies.
Alternatively, the use of soft-lithographic techniques is known. The term soft-lithographic indicates a series of steps suitable for realizing microstructures and nanostructures of high quality. Generally, molds of the elastomeric type are used with predefined channels wherein a liquid penetrates through capillarity and transfer the predetermined pattern to an organic material active layer. As indicated in FIG. 11.